1. Field of Invention
The present invention generally relates to the symbol timing synchronization, and more particularly to the time domain symbol timing synchronization for orthogonal frequency division multiplexing (OFDM) systems under Doppler fading channel.
2. Description of Prior Art
In digital wireless communication, wireless channels will always be considered as fading and frequency selected channels. The effects of these channel impairments eliminate the bandwidth of communication systems. OFDM systems overcome these problems, due to that they transmit different information bits on different sub-carriers. OFDM systems modulate each sub-carrier with different information bits and output an OFDM symbol which is the sum of each sub-carrier. OFDM systems can resist the effect of multi-path reflection and Doppler spread. OFDM systems also have other advantages, such as high spectral efficiency due to nearly rectangular frequency spectrum for high numbers of sub-carriers, simple digital realization by using Fast Fourier Transform (FFT) and Inversed Fast Fourier Transform (IFFT), and so on. Thus OFDM becomes a mainstream technology in digital wireless communication.
Many standards adapt OFDM technology, such as digital audio broadcasting (DAB), digital video broadcasting (DVB), IEEE 802.11/a/g/n, IEEE 802.16, HIPERLAN/2, and etc.
In the wireless channel, the received OFDM symbol may be a timing offset symbol (i.e. there is a delay time between transmission signal and received signal.). If the FFT at receiver end doesn't know when a received OFDM symbol starts, the output data of the FFT are erroneous. In order to avoid the symbol timing offset, the OFDM receiver must achieve symbol timing synchronization to get the precise symbol boundary.
Reliable receiver synchronization is one of the most important issues in OFDM systems. One of the most important synchronization issues for OFDM systems is to know when a received OFDM symbol starts, i.e. the symbol timing synchronization.
The transmission signal of some standards is built up around a transmission frame structure. Such as DAB, the transmission structure contains the synchronization sequence and data sequence. FIG. 1 is a schematic diagram showing the frame structure of the DAB standard. The transmission frame duration is denoted by TF. This transmission frame 10 contains a synchronization sequence 100 and a data sequence 101. The transmission frame 10 consists of consecutive OFDM symbols. The number of OFDM symbols of the transmission frame 10 is dependent on the transmission mode. The synchronization sequence 100 in any transmission mode occupies the first two OFDM symbols of each transmission frame 10.
The synchronization sequence 100 contains two OFDM symbols. Wherein, the first OFDM symbol is a null symbol 1000, and the second OFDM symbol is a phase reference symbol (PRS) 1001. The data sequence 101 consists of consecutive OFDM data symbols 1010. The duration of the null symbol 1000 is TNULL, and the duration of the other OFDM symbols in the transmission frame is Ts. The null symbol 1000 can be used for coarse symbol timing synchronization. The PRS 1001 can be used for channel estimation and fine symbol timing synchronization.
FIG. 2 is a schematic diagram showing the cyclic prefix characteristics of the PRS 1001. The PRS 1001 contains a cyclic prefix 10010 and a phase reference sequence 10011. Wherein, the cyclic prefix 10010 is copied from the last Ng samples of phase reference sequence 10011. The duration of cyclic prefix 10010 is Δ, and the duration of phase reference sequence 10011 is TU. The phase reference 10011 sequence is specified in the DAB standard.
FIG. 3 is a block diagram showing the conventional DAB receiver. This receiver comprises an A/D converter 30, a receiver frontend 31, a FFT 32, a frequency domain symbol timing synchronization circuit 33, and a receiver backend 34. Wherein, the receiver frontend 31 is connected to the A/D converter 30. The FFT 32 is connected to the receiver frontend 31. The frequency domain symbol timing synchronization circuit 33 is connected to the FFT 32. The receiver backend 34 is connected to the FFT 32.
See FIG. 3. The A/D converter 30 receives analog signals from the transmitter and converts the analog signals to digital signals. The receiver frontend 31 receives the digital signals from the A/D converter 30 and performs signal processing, such as filtering of adjacent channel interference, auto-gain control, cyclic prefix removal, clock recovery and so on. The FFT 32 receives the digital data from the receiver frontend 31 and does fast Fourier transform of received digital signals. The frequency domain symbol timing synchronization circuit 33 is used to estimate the symbol boundary of the OFDM symbol and to indicate the FFT 32 when the received OFDM symbol starts. The receiver backend 34 performs signal processing, such as decoding, demodulation, deinterleaving and so on.
The conventional symbol timing synchronization uses the frequency domain symbol timing synchronization circuit 33 to estimate the channel Impulse response. Since the frequency domain symbol timing synchronization circuit 33 obtains the precise symbol boundary from the estimated channel impulse response, the frequency domain symbol timing synchronization circuit 33 then indicates the FFT 32 when the received OFDM symbol starts. Thus the symbol timing synchronization is achieved. The more details about the above symbol timing synchronization are stated next.
See FIG. 3 and FIG. 4. FIG. 4 is a schematic diagram showing the conventional frequency domain symbol timing synchronization. Assume that the received signal without noise r(t)=s(t)*h(t) is the convolution of the transmit signal s(t) and the channel impulse response h(t). In the frequency domain, the Fourier transform of the received signal R(f)=S(f)H(f) is the product of the Fourier transform of the transmit signal S(f) and the Fourier transform of the channel impulse response H(f). By transmitting phase reference sequence 10011 as transmit signal s(t), since the phase reference sequence 10011 is specified in the DAB standard, the Fourier transform of the transmit signal s(f) is known by the receiver. Hence, after frequency domain symbol timing synchronization circuit 33 divides the Fourier transform of the received signal R(f) by the Fourier transform of the transmit signal S(f) and does IFFT of H(f) consequently, the frequency domain symbol timing synchronization circuit 33 can obtain an estimated channel response h(t). See FIG. 4, in this case, the signal constellation of the transmit signal s(t) has no phase rotation. Thus the frequency domain symbol timing synchronization circuit 33 indicates the FFT 32 that the symbol has no offset, and the precise symbol boundary is obtained. If a timing offset signal is received at the receiver end, the received signal r(t)=s(t−t0)*h(t) is the convolution of the timing offset signal s(t−t0) and channel impulse response h(t). Then the frequency domain symbol timing synchronization circuit 33 obtains the Fourier transform of the received signal R(f)=S(f)H(f)exp(−j2 πft0). As showed in FIG. 4, in this case, the signal constellation of the timing offset signal s(t−t0) have a phase rotation 2 πft0 compared to the signal constellation of the transmission signal s(t). Thus the frequency domain symbol timing synchronization circuit 33 obtains an estimated channel response h(t−t0). Finally the frequency domain symbol timing synchronization circuit 33 indicates FFT 32 that the offset time of received OFDM symbol is t0. So the FFT 32 knows that the symbol starts at to, and the symbol timing synchronization is achieved.
The conventional symbol timing synchronization scheme obtains the precise symbol boundary from frequency domain calculation, so the received signal at the receiver end must do the FFT and IFFT once. This scheme needs high computation complexity. In modern wireless networks, high computation complexity can not satisfy the demands of real-time service. Additionally, the effect of the Doppler fading channel may cause the conventional symbol timing synchronization scheme get a wrong symbol boundary.
In order to solve these and other problems as stated above, the embodiment of the invention provides a time domain symbol timing synchronization apparatus and method thereof under Doppler fading channel.